A PABX is an exchange in the private use of a subscriber. The PABX has access to a public switched telephone network through direct trunk lines and direct inward dialling (DDI) lines from an exchange. An intra-PABX call can be established without engaging a trunk line. The direct trunk lines of the PABX are primary trunks of an exchange in the public switched telephone network PSTN, called trunk groups, and access points of the PABX are extensions of the PABX. The PABX also incorporates a switchboard (console) for a telephone operator (attendant) whose task is to handle the incoming calls and serve the extensions. The most important function of the PABX operator is switching: answering the call, connecting, i.e. selecting the desired extension number, and leaving the line. Normally at least part of the incoming call traffic to the PABX is direct inward dialling (DDI) traffic, i.e. each extension has an external telephone number and hence a PSTN subscriber has direct access to an extension without assistance of the operator. This is accomplished in such a manner that a specific DDI code is included in the dialled telephone number. The initial purpose of the DDI function was to reduce the number of exchange calls arriving at an operator's console and thereby to diminish the load on (possibly also the number of) operators. Today, however, operators answer exchange calls only for about 20% of their working time. About 40% of their working time is occupied by picking up DDI calls that are returned to the PABX on the basis of an absence code programmed into the telephone or on the basis of information obtained from a working time monitoring system. The remainder, 40%, of their time the operators pick up calls returned from extensions on account of no reply. Furthermore, the operators receive and deliver a large number of call back messages.
Presently operators of PABXs often have access to an information system from which they obtain the status of the called PABX subscriber (e.g. conference, vacation, sick leave) onto a display unit. The operator can then provide this information to the calling party. The problem with present-day information systems, however, is that the status information stored in them is generally based on information given by the individual himself. Thus updating the status information is dependent on the initiative and care of the individual.
Nowadays PABXs also incorporate voice mail services to which a subscriber for a PABX extension can transfer calls arriving during his absence. The voice mail normally contains a general voice message from the PABX subscriber, prompting the calling party to leave a message in the voice mail. A corresponding function is achieved when a conventional telephone answering machine is used. Also in voice mail, the PABX subscriber himself is responsible for the updating and content of the voice message provided to the calling party, and usually the message contains no information about the reason for or expected duration of the unavailability of the PABX subscriber. Thus the calling party cannot know when the called party will be available or when he will listen to the voice message left by the calling party in the voice mail. In this situation, the only way to check the matter is to make a call to the operator switchboard and enquire the operator. Thus the significance of voice mail in diminishing the work load of the PABX operator will be negligible.
The PABX operators, therefore, will have very little time for answering direct trunk calls which are significant for customer service and which are usually placed by new customers to the company who do not know of any contact person or DDI number of such a person in the company yet. In the worst case, long answering times for trunk calls can even cause the caller to hang up, in which case the new customer contact is completely lost. If the work load of the PABX operator could be diminished in other respects, there would be more time to answer the direct trunk calls and service would improve in this respect.